1. Field of the Invention
The present invention relates generally to profile extrusion die hardware. It relates particularly to a method for designing a profile die for polymer extrusion.
2. Description of the Related Art
Extrusion is the process of forcing material through a die having an extrusion profile, to form a product having a cross section that matches the extrusion profile. The length of the extruded product is determined by the amount of material forced through the die. The cross section of the extruded product is determined by the shape of the profile, which is defined by the die designer.
The design of flat plate dies (also called profile or orifice dies) is affected by many variables, including the composition of the viscoelastic extrudate, as well as the operating parameters of the extruder machine. For low Reynolds Number flows of viscoelastic polymers, the resulting extrudate can swell or shrink anywhere from 10% to 300% depending upon the die shape, material composition, extruder screw speed, temperature, etc. Chris Rauwendaal states in Polymer Extrusion (published by Hanser, New York, 1986), “A good die designer must anticipate the amount of uneven swelling, and design the flow channel accordingly. This process is a very difficult task and the determination of the flow channel geometry is often done by a ‘trial and error’ process. Determination of the proper flow channel geometry to minimize uneven swelling by engineering calculations is generally not practical.”
The objective of the subject invention is to present a new method for designing profile extrusion dies. The method is based on an analogy that exists between membrane deflection under pressure and low Reynolds Number pressure driven fluid flow die swell. Sir Horace Lamb (Hydrodynamics, Dover, New York, 1932) comments on an analogy between the theory of the steady motion of a viscous liquid in two dimensions and that of the flexure of an elastic plate. However, in spite of the fact that this analogy was recognized many years ago, and designing profile dies for extrusion of rubber and other materials has been a problem for even longer, to date no one in the art has recognized that this analogy might beneficially be applied to polymer extrusion dies where swelling or contraction is a problem.
Die designers typically use experience-based rules to derive profile extrusion die designs. After designing an initial profile extrusion die, the design is implemented in a trial extrusion process. The designer evaluates the resulting extrusion to determine whether the die design produced the desired result. If not, then the die dimensions are modified and tested in another extrusion process. Often, many iterations of modifying and testing the die are necessary before arriving at a design that yields the desired result. This current methodology is time-consuming and requires a great deal of experience-based judgement and die modification. In addition, the designer must modify the die and re-initiate the process every time product dimension requirements change, or when a new material is to be extruded. Much of the problem is caused by the fact that the extruded material stores energy while under pressure, and then swells as it exits the die. The expansion of the extrudate is generally unpredictable for complex profile geometries, and may vary from 10% for some polymers to 100% for some rubber materials. Consequently, extrusion die design is often slow and costly, and designers resist the introduction of new materials to the profile extrusion market.
It would be of great benefit to the extrusion industry to provide a method which would decrease much of the iterative guesswork and reliance on experience currently required by the die design process. Die designers would prefer for profile die design to be less of an art, and more of a science. To that end, there have been some significant developments in the past few years, but nothing that has fully solved the iterative design problem, and nothing of the nature of the subject invention.
Recent inventions by Wang et al and Huang, et. al., particularly have attempted to address this problem. Patents by Wang (U.S. Pat. Nos. 5,608,637, 5,926,393, and 6,246,918) describe methods by which a slit die is used to determine properties of particular polymer resins, information sheets are derived from the acquired properties, the information is stored in a computer, and the data is used to design a profile die. Patents by Huang (U.S. Pat. Nos. 5,974,850 and 6,153,131) describe the design of pockets in thick dies to increase extrusion speed, and the development of extrusion processes for existing profiles, given the properties of the resin to be extruded. To date, no inventions successfully address the problem of designing an extrusion profile based on polymer properties and desired product dimensions, such that die swell for complex profile shapes can be effectively predicted.